New findings by the Principal Investigator indicate that HIV-1 infects human neutrophils. Results suggest that infection occurs in cis and by a CD4-independent mechanism. Infection is observed after incubation with either X4 or R5 strains of HIV-1, although neutrophils are CCR5-negative and express only limited amounts of CXCR4 on their surface. Thus, we postulate that HIV-1 infects neutrophils by a potentially novel mechanism. Moreover, these findings indicate that neutrophils may constitute a previously unrecognized target for HIV-1 infection that, in turn, may contribute to the transmission and dissemination of HIV-1 infection as well as to the complications of HIV-1 disease. Specifically, neutrophils comprise approximately 60% of circulating leukocytes and are also critical effector cells in host defense against bacterial and fungal pathogens. As such, neutrophils may be a target for infection following intravenous exposure to HIV-1 as well as contribute to HIV-1 transmission at mucosal sites. The mucosa is the natural portal of HIV-1 entry in the transmission of infection, and neutrophils are constitutively present at mucosal sites and accumulate further in response to mucosal infection. In the absence of mucosal lesions, infection is dependent on the translocation of HIV-1 across the epithelial border, a process that is markedly enhanced by exposure to cell-associated virus. Thus, infected neutrophils in semen or breast milk could facilitate the transmission of HIV-1 infection within the mucosa. Lastly, infection of neutrophils may contribute to alterations in neutrophil function that, in turn, may contribute to the increased incidence of mucosal infections or, in some cases, to epithelial damage in HIV-1-positive individuals. Therefore, this project will determine: (1) if cis-infection of neutrophils by HIV-1 leads to producton of replication competent virus; (2) the mechanism for HIV-1 entry into neutrophils; and (3) whether neutrophils of HIV-1-positive individuals are infected in vivo by the virus. It is predicted that the results of this study will provide a basis for expanded and detailed analysis for potential and previously unrecognized involvement of neutrophils in HIV-1 transmission and disease.